Extending WiFi to one mile, thanks to empty TV channels

Rice University grad student Ryan Guerra is on a mission to extend the range of WiFi signals from a few hundred feet to a mile—and beyond. This month, he succeeded, thanks to some nifty engineering and a few empty TV channels.

The first beneficiary of his work is Houston resident Leticia Aguirre, 48, who lives at the very edge of a local free WiFi network run by Technology for All. The high frequencies (2.4 GHz and 5 GHz) used by WiFi mean that signals don't easily penetrate the tree branches and leaves which surrounded Aguirre's home.

On the "deadest, stillest day of winter" the signal might be reliable, Guerra told me, but most of the year, it has been frustratingly intermittent. Even though the WiFi connection was free and Aguirre can't afford DSL or cable, her experience was so poor she considered canceling the service.

Guerra decided to use Aguirre's home as the first location for new "Super WiFi" test gear being developed and tested at Rice. Instead of relying on traditional WiFi frequencies, the Super WiFi project downshifts the signal into an empty TV channel—in this case, channel 29, which has the additional virtue of having empty adjacent channels, as well.

Guerra was put in charge of creating the Super WiFi gear for Aguirre's home, which was one mile from the Technology for All transmission tower. He began with an off-the-shelf 2.4GHz WiFi card on a computer running Linux. The card's output is piped through a frequency translator prototype from Alcatel Lucent, which shifts the signal down to Channel 29's 563MHz—far better for plowing through trees and walls.

Guerra installing the Super WiFi antenna

While WiFi channels at 2.4GHz are 20MHz wide, TV channels only have 6MHz of bandwidth, so this setup also has to squeeze the incoming signal down to 5MHz of bandwidth in order to stay safely within channel boundaries (in the future, techniques like channel bonding should increase the available bandwidth). Output from the translator then goes to a small TV antenna on Aguirre's home, where it's sent to the local Pecan Park transmission tower and patched into the fiber backbone connection there.

The setup "actually works very well," says Guerra. The longest link he could make with existing point-to-point WiFi connections was 400-500 feet; with the new Super WiFi gear in the TV band, he can reach a mile—and it's not a point-to-point signal. Instead, the transmitter serves up a directional 60 degree beam, and anyone in its path can receive broadband service.

The wider signal beam also has a side benefit: smaller antennas. Aguirre used to require an antenna mounted on a 30 foot pole in order to get a line-of-sight, point-to-point connection; the newer setup doesn't need this kind of tight alignment, and so Aguirre now uses a much more discreet TV antenna.

Results so far have been good. Rice has worked with mesh WiFi networks for years, but researchers have noted that the quality of channels varies dramatically over time. The new connection has been quite stable, even with leafy trees and through bad weather, and has yet to show any interference or connection problems, even at a mile from the transmitter.

From left to right: UHF-band filter, frequency translator, PC motherboard and WiFi card, power supply

The current approach has some bandwidth limitations. Because it uses existing WiFi protocols but uses only 25 percent of WiFi's bandwidth, Guerra's setup will never get more than 25 percent of WiFi's maximum throughput—and indeed, that's what he's seeing.

The greater range of Super WiFi gear means that the current setup will also run into problems as it scales up. WiFi uses a Carrier Sense Multiple Access (CSMA) approach to sending data, which means that WiFi transmitters try to detect other nearby transmitters and wait for them to fall silent before sending data of their own. This works pretty well when only a few devices coexist, but it can become chaotic when hundreds of nodes are involved.

As Guerra rolls out his Super WiFi solution to other Pecan Park residents, he anticipates running into CSMA congestion, which will lower the efficiency of the network. Eventually, whole new protocols may be required, probably relying on scheduled access mechanisms in which transmitters are assigned specific time slices wherein they can transmit without having to detect other signals (cell networks often work this way, and companies like Microsoft are already researching new protocols for use in the TV white spaces).

But the work has shown that empty TV channels will be a huge boon for broadband. Urban areas will likely rely on wireline networks, but Guerra sees terrific potential for people like his rural relatives, who need better last-mile connections. If Super WiFi can easily reach a mile in urban conditions, it's likely to go further in rural locations.

Guerra's work doesn't just involve time spent at the lab bench or with computer models, giving him a unique grad school experience. "I didn't realize how unique it was until I started going out in the field and installing equipment," he says, until he would return from a trip to see other grad students looking up in jealousy from their computers.